High frequency (W-band, 95 GHz) EPR spectroscopy was applied in order to determine very accurately the spectroscopic parameters (g and a-tensors) distinguishing the free radicals formed from rat hemoglobin by methemoglobin-generating agents and trapped by DMPO. The high resolving power of the W-band instrument makes it possible to recognize the signature of each different trapped free radical . Since preliminary results have been successful, we are now examining various DMPO spin adducts.